If my gf did what yours is doing I'd simply let her do it, but there would be absolutely no contact between us that day, I'm too busy you see

If she came back telling me what a great guy he is and her phone is full of texts from him I'd dump her on the spot and watch her attitude quickly change

If she came back clearly not interested in him and telling me how beta he is I'd reward her in some way

I don't like the advice given out earlier in this thread.

@ darkone: while the information you give is sort of correct, your remedy is weak shit: you won't let your girlfriend hang out with a guy one on one, there has to be somebody else around? Come on. You're admitting your own weakness. You're afraid she will give in because there are so many men of higher value around that you can't risk her being alone with a guy. I know that's not what you explicitly think, but that's what it signals.

I would let your girl do whatever she wants. If she cheats, too bad, it couldn't be helped. Forbidding her to go will never do anything good. See those posts:

cheating-male-best-friends-and-jealousy ... highlight=

and this:

http://www.solvemygirlproblems.com/2010 ... tionships/

and this:

http://www.solvemygirlproblems.com/2012 ... et-forces/

My girlfriend has one on one time with guys all the time. I'm totally OK with it. What do I care? I don't even think about the fact that she might cheat for one second. She goes shopping with them and that sort of stuff. She once baked brownies with a guy, FOR me. That poor guy was spelling MY name with those sugar letters on a brownie. How threatening can all this be?

Seriously, read those posts well.